This invention relates to the field of printing, and in particular to a method and apparatus for mounting flexographic or letterpress plate segments onto a printing plate carrier.
Flexography and letterpress use printing cylinders on which printing plates are mounted using different mounting methods. When printing in color, one cylinder is used for each color, i.e., for each color separation.
Conventional printing uses a single printing plate on the cylinder covering the whole area to be printed. An alternate is using a plurality of segments of plate material mounted on the cylinder. Such a method uses less printing plate material than using a single plate covering the whole area to be printed, and is suitable for flexography and letterpress because flexographic and letterpress plate material is relatively expensive. An economical method for creating plate segments, each containing register marks is described in U.S. patent application Ser. No. 09/946,145 to Klein, et al., titled xe2x80x9cMETHOD, APPARATUS, AND COMPUTER PROGRAM FOR REDUCING PLATE MATERIAL WASTE IN FLEXOGRAPHY.xe2x80x9d
A known manual method for mounting the plate segments onto a carrier to produce a cylinder with multiple plate segments includes first producing flat printing plate sheets, cutting the plate segments, and manually mounting the segments onto a printing plate carrier such as a drum, sleeve, or a mylar sheet. Several tools exist to facilitate this operation, e.g., so that the operation is more repeatable. Success is highly dependent on the skill of the operator.
The manual method includes applying glue to the back of the printing plate segments or making the printing plate carrier adhesive, e.g., by applying glue or by using double sided adhesive tape. The manual method further includes the operator manually aligning register marks on the printing plate segments with marks that have been provided by a mounting apparatus. The operator can use a prior-art mounting apparatus for this. The apparatus helps registering by using a half-transparent mirror or a video screen to display a register mark on top of the image of the plate segments. In all such procedures, an operator manually decides where to mount the plate segment. As a result, the quality of the resulting printing cylinders may vary depending on the skill of the operator. Furthermore, the mounting may take a relatively long time, especially when high precision is required. Furthermore, large plate segments are more difficult to mount than smaller plate segments, mainly because it is difficult for human operators to handle large plate segments.
An apparatus that help a human operator to mount printing plate segments onto a printing cylinder is called a manual mounting machines herein, and the mounting method is called a manual mounting method.
Another prior art method for producing cylinders that include printing plate segments produces the printing cylinders directly. Blank, unimaged plate segments are applied on a cylindrical plate carrier, typically a sleeve, and imaged in a computer-to-plate drum imaging device such as the Esko-Graphics Cyrel Digital Imager (Esko-Graphics NV, Gent, Belgium, the assignee of the present invention). The imaging device is used to directly expose the sleeve carrying the flexographic plate segments. After exposure, the cylindrical plate carrier with the exposed plate segments attached is moved away from the imaging device and processed in round washing equipment. Because the printing plate segments are not removed from the sleeve or printing cylinder for the processing, the image register is maintained throughout the process until printing. The advantage of this method over the more conventional method of manually mounting imaged segments is the improved register and decreased mounting cost. The second method however is less popular than the manual mounting method, mostly because of cost. Producing imaged flat flexographic plate sheets for is relatively inexpensive because the equipment is widely available and its cost mostly amortized. There is substantial investment required for exposing flexographic plates xe2x80x9cin the round,xe2x80x9d i.e., on blank segments mounted on cylindrical carriers.
One of the problems with using a plurality of segments is accurate registration. As a result, imaging on a single sheet is still often used in flexography, even at the cost of the wasted plate material.
Thus there is a need in the art for an apparatus and method of producing print-ready cylinders by first exposing flat plate materials and cutting the flat plates into segments, while maintaining register accuracy that does not depend on the skill of the operator to the same extent as the known manual method. There further is a need in the art for a mounting device that provides for mounting exposed plate segments onto a plate carrier while maintaining accuracy, such a device being considerably less expensive than a full xe2x80x9cin the roundxe2x80x9d processing unit.
Described herein is a method and an apparatus for mounting printing plate segments onto a printing plate carrier. In one embodiment, the printing plate carrier is on a cylindrical drum, i.e., is a cylindrical plate carrier, while in another embodiment, the carrier is a sheet laid out on a substantially flat surface that, after mounting, can be placed on a drum.
The method includes accepting positioning data indicative of a set of first positions and loading the imaged segments onto a working surface of the loading table at approximately the corresponding first positions. The method further includes, for each segment, detecting the position of the segment on the table, and using a mechanical pick-up system to pick up the segment and to carry the segment to a final position on the carrier. The carrying is via a path determined using the detected position on the table and the final position. In one embodiment, the table""s working surface has a set of vacuum holes coupled to a vacuum system. The mechanical pick up system includes a plurality of suction caps coupled to the vacuum system to pick up the plate. One version includes a mechanism for cutting the segments from the sheet.
In one embodiment, the mechanical pick-up system includes one or more pick-up arms. Each arm is rotatably connected to the loading table and has one or more suction caps coupled to the vacuum system. For example, one version has four pick-up arms. Each arm has a head that is movable lengthwise along the arm. The head includes a video camera and a single suction cap coupled to the vacuum system.
In one embodiment, each imaged plate segment has a plurality of register points. The detecting of the position of the segment detects the register marks. The detecting uses the video detecting system, e.g., the cameras mounted on the arms of the mechanical pick-up system in the version that has such arms. The picking up includes positioning the pick-up system such that the suction caps are close to the detected positions of the register marks, such that the picking up is at pick-up points close to register marks. The carrying of the segment includes initially rotating the cylindrical carrier so that the location on the carrier of the roll-off point of the segment is such that the mechanical pick up system can place the roll-off point of the segment thereon. The roll-off point is a point on the final position of the segment. The method includes carrying the segment such that the segment""s roll-off point can be placed on the location on the carrier of the roll-off point, placing the segment""s roll-off point onto the roll-off point location on the carrier, and placing the remainder of the segment on the carrier by translating the mechanical pick-up system and simultaneously rotating the cylinder such that the segment ends placed at its final position.
In one version, no deformation of the segment can occur during the carrying. In another version, the method includes picking up each plate segment at more than two independent points and carrying each segment to a respective final position on the carrier, such that some of the plate segments may need to be deformed to reach their respective final position. For a plate segment that is so deformed, the final position cannot be reached by picking up the plate segment at only two points because of the required deformation of the plate.
The apparatus includes a computer system to control the various elements and functions such as the motion of the mechanical pick-up system, the detection of position by the video detection system, and the determining of the path for carrying the plate segments.